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Q: How can there be space for so much CO2 underground?

A. Trillions of tiny holes make a big space...like a rock-hard sponge.

Underground is not a cave or a balloon. It is made up of layers of different types of rocks. The kind of rock that CO2 will be stored in is porous. This means the rock is made up of small grains stuck together, and between those grains are tiny spaces.

Think about a jar packed full of marbles. There is space between the marbles. If you had a liquid, you could pour the liquid into the jar and it would fill the space between the marbles.

This is similar to how underground reservoirs for CO2 work. The marbles are like the grains of the rock. The CO2 that is stored underground is in a compressed phase that flows like a liquid. The CO2 fills the pores by displacing the salty water called brine between the grains of the rock.

Now imagine that you didn’t just have one jar of marbles. Imagine you had trillions of jars of marbles extending over many square miles of land that is many 100's of feet thick. You could store a lot of CO2!

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A. Underground reservoirs are huge places.

The size of a typical underground reservoir is about 20 trillion cubic feet. (A jar the size the student is holding in this photo is about half of a cubic foot.) That’s a lot of capacity.

Just how much CO2 a particular reservoir can hold is something scientists are working to figure out because many different factors besides volume influence a particular reservoir's capacity. Each of these factors is itself an area of active research, and they include:

Porosity - a measure of the space between sandgrains.

Permeability - how easily carbon dioxide can get into the space between sand grains

Reservoir volume - the total size of the useable reservoir

Efficiency of use - how much of the space you can actually use for storing CO2.

Confining system - the area where an appropriate geologic seal traps the CO2

Location of fractures - places where carbon dioxide could move from one geologic formation to another

Scientists know that there's a lot of space underground. Scientists estimate that 645 Gigatons of CO2 could be stored in the Gulf of Mexico alone. Researchers at the Gulf Coast Carbon Center are currently evaluating and refining those estimates. For reference, the International Energy Agency reports that in 2009 U.S. emissions of carbon dioxide were about 5 Gigatons.

The Gulf Coast Carbon Center's Offshore Miocene Repository project is developing an assesment of the capacity of the geologic reservoirs deep under the Gulf of Mexico. You can read more about it on their website.